The present invention relates to a method of exploration of a medium by the transmission of energy into this medium, in particular the geophysical prospecting of the ground medium by the creation of seismic waves permitting of the simultaneous obtaining several distinct seismic profiles of a selected geological area.
When an energy pulse is emitted from an emission point, the mechanical waves generated in the ground follow various paths, being subjected especially to reflection on reflectors which are the interfaces of geological strata of different constitutions. Appropriate pick-up transducers, such as seismographs, receive the reflected waves; these pick-ups can be grouped and distributed so as to have one or more reception points available.
Seismic prospecting of the ground makes use of emission sources which are either an explosive disposed in a tubed or untubed hole, or a source operating on the surface, such as for example, a dropping weight or the like. In the latter case it is very difficult to synchronise the sources when several of them, whether identical or not, are actuated simultaneously. Furthermore in order to obtain very precise information one must record on very close lines, and even on a crossing grid of lines. Much time is necessary to arrange the seismographs and, on the one hand, to effect the emissions and, on the other hand, to effect the recordings over all the lines of the grid, especially if it is desired to close up the meshes of the grid in order to determine the maximum information in a selected area.
In order to be able to record along several lines at the same time, the improvement as described in U.S. Pat. No: 3,506,955 (Backus and Schneider) has been proposed. This improvement consists in disposing two or three emission sources in relation to a single receiver formed by a line of seismographs and interposed between two sources, in successively emitting energy pulses with the aid of each of the sources, in receiving the reflected waves on the single receiver and in recording these reflected waves so that they can subsequently be processed and the desired data can be deduced from them. The pulses emitted by each of the sources are separated by a time interval equal at least to the maximum time T in which one is interested so that the waves created by each of the sources can never interfere during this time.
In the case where there is a large number N of sources, it follows that each of the said sources operates again only after a time NT, and that the exploration of a geological stratum of a given area necessitates the use of the apparatus for carrying out the method for a relatively long time. The price per kilometer of the section of the geological stratum prospected, which is a function of the speed of travel of the apparatus for carrying out the method, therefore becomes prohibitive.